1. Field of the Invention
The present invention relates to a semiconductor laser device with window regions, having a buried active region.
2. Description of the Prior Art
In recent years, semiconductor laser devices having advantages such as a small size, high output, and low price put to practical use. In the past, the use of a laser beam as a light source in general industrial and consumer equipment has been difficult. However, owing to the above-mentioned tendency, the application of laser devices to general industrial and consumer equipment have been increasing. It is expected that semiconductor laser devices will be applied in further various fields in the near future. Under the circumstances, semiconductor laser devices used as a light source for optical disk apparatuses are required to emit laser light with high output and to give high reliability.
It has been known that the high output operation of semiconductor laser devices cause defects due to the deterioration of crystals at cavity end facets in an active region. The reason for this is that a laser beam is absorbed by a surface state in the end facet to generate heat, and cause the crystal dislocation in the end facet. In order to overcome this problem, a semiconductor laser device with window regions is proposed. In this type of device, an active region including an active layer is buried in layers having a bandgap larger than that of the active layer.
A conventional typical semiconductor laser device is shown in FIG. 5 (Japanese Laid-Open Patent Publication No. 58-159388). FIG. 6 shows a cross-sectional view of the device taken along a line C-C' of FIG. 5. This semiconductor laser device is fabricated, for example, as follows:
As shown in FIG. 5, on an n-GaAs substrate 21, an n-AlGaAs cladding layer 22, an n-AlGaAs auxiliary cladding layer 23, a GaAs active layer 24, a p-AlGaAs auxiliary cladding layer 25, a p-AlGaAs cladding layer 26 and a p-GaAs contact layer 27 are successively grown by an appropriate crystal growth technique. Then, this layered structure is formed into a mesa-stripe by an appropriate etching technique, after which side faces and end facets are filled with an AlGaAs layer 28 with a bandgap larger than that of the active layer 24. At this time, the cladding layer 22, the auxiliary cladding layer 23, the active layer 24, the auxiliary cladding layer 25 and the cladding layer 26 form a Separate Confinement Heterostructure (SCH).
Moreover, a Si.sub.3 N.sub.4 film 29 is formed on the contact layer 27, and then a portion of the Si.sub.3 N.sub.4 film 29 positioned above the mesa-stripe is removed by etching to form a current injection path. Finally, a p-sided electrode 30 is formed on the exposed contact layer 27 portion and on the Si.sub.3 N.sub.4 film 29 as shown in FIG. 6, and an n-sided electrode 31 is formed on the back surface of the substrate 21, thereby producing a semiconductor laser device with a window region as shown in FIG. 5. In the end facets of the active region, the layer 28 constitutes window regions, so that light is prevented from being absorbed by the surface state, and reliability can be remarkably improved.
However, in the above-mentioned conventional semiconductor laser device with window regions, when the layered structure is etched to form a mesa-stripe, a depth, i.e., the height of the mesa-stripe cannot be regulated because of the lack of means for limiting the etching depth. Light is reflected on the interface between the substrate 21 exposed by etching and the layer 28 to influence the properties of emitted light; however the properties of the emitted light cannot be regulated since the position of the interface cannot be specified. In the window regions, there are no waveguide structures for guiding a laser beam in vertical and horizontal directions. Because of this, the conventional semiconductor laser device with window regions has the following problems:
(1) Since the waveguide structures are not formed in the window regions, proportion coupling efficiency that light emitted from the internal waveguide in the active region is reflected on the end facet and fed back by coupling to the internal waveguide is small and the required laser gain cannot be obtained, leading to an increase in the threshold current.
(2) In the case where the etching depth for forming a stripe is small and an interface between the layer 28 and the substrate 21 is positioned in the vicinity of the active layer (as close as about 0.2 .mu.m or less), and the thickness of the layer 28 is small, a laser beam is absorbed by the substrate 21 and the contact layer 27, causing distortion of a far-field pattern due to the waveguide loss and wave front distortion.
(3) Since there is no waveguide structure in a horizontal direction and there is wave front distortion in a vertical direction, positions of a beam waist of the window region waveguide in the horizontal direction and that of the window region in the vertical direction do not correspond to each other. This causes astigmatism.